1. Field of the Invention
This invention relates to a cooling method for a fuel cell in which heat produced during power generation in the fuel cell is dissipated by circulating a cooling fluid and by using a heat exchanger.
2. Background Art
Some fuel cells mounted in fuel cell vehicles are formed by stacking a plurality of fuel cell units, each of which comprises: a solid polymer electrolyte membrane, such as a solid polymer ion-exchange membrane electrode or the like, sandwiched between an anode and a cathode; and a pair of conductive separators holding the solid polymer electrolyte membrane therebetween, wherein a fuel gas, e.g., hydrogen gas, is supplied to the anode of each of the separators and an oxidizing gas, e.g., oxygen-containing air, is supplied to the cathode of each of the separators so as to generate electric power. In this type of fuel cell, hydrogen ions produced at the anode by a catalytic reaction move to the cathode through the solid polymer electrolyte membrane, and react with oxygen at the cathode to generate electric power.
In this type of fuel cell, because heat is produced during power generation and because the temperature of the fuel cell should be maintained within an appropriate range, the fuel cell is cooled by flowing a cooling fluid through cooling fluid passages formed in the separators of each of the fuel cell units and by dissipating heat of the cooling fluid using a heat exchanger. In a case in which such a cooling system is employed, it is necessary to control the amount of heat dissipated in order to avoid excessive cooling of the fuel cell when the fuel cell is operated in a cold environment, or is being operated so as to generate a small amount of power. As one conventional heat dissipation control method, a method in which cooling fluid circuits are switched depending on temperature using a thermostat valve is known. In this method, the cooling fluid circuits are switched so as to circulate the cooling fluid in the fuel cell while detouring the cooling fluid around the heat exchanger when the temperature of the cooling fluid is in a low temperature zone, i.e., below a temperature at which the thermostat valve operates (hereinafter, this temperature is referred to as the thermostat operating temperature), and so as to circulate the cooling fluid in the fuel cell while also circulating it through the heat exchanger when the temperature of the cooling fluid is in a high temperature zone, i.e., above the thermostat operating temperature.
When such a cooling system in which the separators are directly cooled by the cooling fluid is employed, because the conductivity of the cooling fluid must be maintained to be low in order to prevent electrical leakage via the cooling fluid, the cooling fluid is made to flow through an ion exchanger or the like to remove ions contained in the cooling fluid so that the conductivity of the cooling fluid is maintained to be low.
However, in the system in which heat dissipation amount is controlled by switching the cooling fluid circuits using the thermostat valve, because the cooling fluid sits in the heat exchanger and in passages for circulating the cooling fluid through the heat exchanged when the cooling fluid circulates while detouring around the heat exchanger at low temperature, the conductivity of the sitting cooling fluid may be increased due to ions dissolved from the heat exchanger or the passages. If the conductivity of the cooling fluid sitting in the heat exchanger or the passages is increased under low temperature conditions, the cooling fluid which has been sitting in the heat exchanger and which has a high conductivity may flow into the fuel cell when the cooling fluid circuits are switched upon completion of warm up.
Conventionally, in order to avoid such a problem, the heat exchanger and circulation pipes for the cooling fluid were made of material from which only a small amount of ions may dissolve; however, the material restricts the shape or manufacturing method of the heat exchanger, whereby the heat exchanger could be large, heavy, and expensive. Alternatively, the inside of the heat exchanger or the like may be coated in order to suppress ion dissolution; however, ions may dissolve when the coating is degraded.